MyBB Community Forums

its easy...

find template you want to change:
1. index page templates (index) --> for Index
2. forum display templates (forumdisplay) --> for forum or thread list view
3. show thread templates (showthread) --> for post or thread view

as explain above:
find {$header}
start place html table code after that.

and the end of table code must place before {$footer}
I hope you have basic html code knowledge
http://www.w3.org/TR/CSS21/tables.html

here the simple code

{$header}
<table><tr>
<td>left side ads code here</td>
<td>
--bla-bla-bla original code (don't touch)-- will be display on the right side
</td>
</tr></table>
{$footer}

Thats it!